The present invention relates to methods for producing substantially round, spherical and sintered particles from a slurry of an alumina-containing raw material. The methods described herein provide for nucleation and promotion of the growth of mullite crystals in the sintered particles, which in turn contributes strength to the sintered particles. The sintered particles are prepared from a slurry of a calcined, uncalcined, or partially calcined raw material having an alumina content of more than 55% by weight, and a mullite growth promoter in an amount of from about 2 to about 10% dry weight of the total solids in the slurry. The sintered particles produced according to methods described herein are suitable for use as a propping agent or as a foundry media.
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Permeability of the formation is essential to permit oil and gas to flow to a location where it can be pumped from the well. Sometimes the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the permeability of the formation drops to the extent that further recovery becomes uneconomical. In such cases, it is necessary to fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Such fracturing is usually accomplished by hydraulic pressure, and the proppant material or propping agent is a particulate material, such as sand, glass beads or ceramic particles, which are carried into the fracture by means of a fluid.
The foundry industry has traditionally used inexpensive natural sands for casting. However, natural sands exhibit large thermal expansion characteristics, which means that the core boxes and molds must be highly pre-engineered to allow for the expansion of the sand when it comes in contact with the heat from the molten metal. Many times, despite pre-engineering efforts, expansion related defects occur resulting in scrapped parts or parts requiring high cleaning cost to correct. In addition, natural sands contain crystalline quartz silica, which is a health and environmental hazard within the workplace. Also, natural sands are mined, which results in product inconsistencies that lead to casting inconsistencies. Furthermore, most silica sands are not well rounded and are somewhat friable resulting in low flowability and compaction, high breakdown and high solid waste emissions. Specialty sands, such as zircon and chromite, have low expansion properties but are very dense, making core handling more difficult and requiring more resin to obtain adequate tensile strengths. These products are mined and present concerns of availability, consistency and sizing flexibility. They also tend to be somewhat friable.